1. Field of the Invention
The present invention relates to portable gas sensing devices of the type used to monitor atmospheric levels of selected gases in a combustible environment. More particularly, the invention relates to a portable gas sensor equipped with a fault protective battery cap to interrupt flow of battery current exceeding a preselected threshold level.
2. Description of the Prior Art
In many drilling, mining, and industrial situations, the atmosphere may occasionally contain gases which could prove hazardous to workers in the area. It is therefore often desirable in these situations that the workers carry a portable gas sensor for continually monitoring the atmospheric level of selected gases. If the concentration of the monitored gas rises above a desirable level, the worker will be alerted so that appropriate action can be taken. Preferably, these gas sensors are battery powered and small enough to be conveniently worn, for example, on the worker's belt.
Such portable gas sensors, as well as other instruments designed for use in locations where flammable or combustible material may be contained in the ambient atmosphere, should preferably meet a design standard referred to as "intrinsically safe." An instrument is referred to as "intrinsically safe" if it normally, or in specified fault conditions, will not produce a spark or other thermal action sufficient to cause ignition of the flammable or combustible material when mixed with air in its most easily ignitible concentration. Various agencies, such as Underwriters Laboratories, Factory Mutual Research, Canadian Standards Association and Cenelec, certify instruments as meeting or failing to meet this criteria.
Instruments having a design designated to be "intrinsically safe" generally incorporate current limiting elements to restrict current flow to levels which are considered acceptable in the event of certain internal faults. In portable instruments, fuses are commonly used to reduce the otherwise impracticable size and cost of other necessary current limiting elements such as resistors. A drawback in using a fuse, however, is that, in most cases, the fuse itself must be considered "intrinsically safe" such that any arc occurring from failure of the fuse element must be incapable of igniting a hazardous atmosphere. Therefore, the fuse generally must be encapsulated within a material that provides a gas tight seal about its body.
A common type of fuse utilized for this purpose is a commercially available "safety fuse" which resembles a large leaded resistor. In addition to being somewhat costly, this type of fuse is difficult to implement in a small portable instrument. "Intrinsically safe" fuses have also been constructed by encapsulating a fuse inside of a replaceable unit which also contains batteries used to power the instrument. This implementation has caused problems in that servicing of the instrument has typically resulted in failure of the fuse. As a result, replacement of the entire battery pack unit has often been necessitated even though the batteries frequently had remaining useful life.